Uncatalyzed [4 + 2] cycloadditions of 3-nitrocoumarins with vinyl ethers in solventless conditions. A new entry to chromene derivatives

The [4 + 2] cycloadditions of 3-nitrocoumarins 5 with electron-rich dienophiles (ethyl vinyl ether (8), 2,3-dihydrofuran (9), and 3,4-dihydro-2H-pyran (10)) were investigated in water, in neat conditions, and in organic solvents. The cycloadditions do not require the use of catalysts and are highly endo diastereoselective, and in water the cyclic nitronates 13, 18, and 23 are converted into chromene derivatives via hydrolysis, decarboxylation, and acetalation reactions. A one-pot procedure based on consecutive reactions in neat/water conditions allows 3-nitrocoumarins 5 to be used as building blocks for the synthesis of chromanols and tetrahydrofuro- and tetrahydropyranochromenes. For the first time, the hydrolysis of cyclic nitronates having the C-O bond of 1,2-oxazine ring as a part of an acetal was investigated.